This invention relates generally to membranes and their use and, more particularly, to oxidative conversion membranes which permit the selective conveyance of a form of oxygen therethrough and which membranes promote oxidative conversion reactions.
Natural gas is a primary chemical feedstock used in the manufacture of numerous chemicals, such as methanol, ammonia, acetic acid, acetic anhydride, formic acid, and formaldehyde, for example. Furthermore, as the uncertain nature of the limited supplies of and access to crude oil has become increasingly apparent, alternative sources of hydrocarbons and fuels have been sought out and explored. The conversion of low molecular weight alkanes (lower alkanes) to higher molecular weight hydrocarbons has received increasing consideration as such low molecular weight alkanes may be generally available from more readily secured and reliable sources. Natural gas, partially as a result of its comparative abundance, has received a large measure of the attention that has focused on sources of low molecular weight alkanes. Large deposits of natural gas, mainly composed of methane, are found in many locations throughout the world. In addition, low molecular weight alkanes are generally present in coal deposits and may be formed during numerous mining operations, in various petroleum processes, and in the above- or below-ground gasification or liquefaction of coal, tar sands, oil shale and biomass, for example.
Today, much of the readily accessible natural gas generally has a high valued use as a fuel whether in residential, commercial or in industrial applications. Additional natural gas resources, however, are prevalent in many remote regions of the world, such as remote areas of Western Canada, Africa, Australia, USSR and Asia. Commonly, natural gas from these remote resources is referred to as "remote natural gas" or, more briefly, "remote gas".
In many such remote regions, the widespread, direct use of natural gas as a fuel is generally not currently profitable or economical. Further, the relative inaccessibility of natural gas from such resources is a major obstacle to the more effective and extensive use of such remote gas as the transportation of the remote gas to distant markets wherein it could find direct use as a fuel is typically economically unattractive.
The dominant technology currently employed for the utilization of remote natural gas involves conversion of natural gas to a liquid form via the formation of synthesis gas, i.e., a process intermediary composed of a mixture of carbon monoxide and molecular hydrogen, generally having a hydrogen to carbon monoxide molar ratio in the range of about 1:5 to about 5:1 and commonly referred to as "syngas". Synthesis gas has utility as a feedstock for conversion to alcohols, olefins, or saturated hydrocarbons (paraffins) according to the well-known Fischer-Tropsch process, and by other means. When hydrocarbon products such as gasoline and diesel fuel are sought, the syngas can be converted to syncrude, such as with Fischer-Tropsch technology, and then upgraded to the desired transportation fuels using typical refining methods. Alternatively, syngas can be converted to liquid oxygenates which can be blended with conventional transportation fuels to form materials such as gasohol, used as an alternative fuel or converted to transportation fuels by catalyst such as certain zeolites.
Synthesis gas is not a commodity; rather, it is typically generated on-site for further processing.
Prior methods for producing synthesis gas from natural gas (typically referred to as "natural gas reforming") can be categorized as:
a) those relying on steam reforming wherein natural gas is reacted at high temperature with steam and wherein the heat required for the reforming is supplied externally; PA1 b) those relying on partial oxidation in which methane is partially oxidized with pure oxygen by catalytic or non-catalytic means and wherein the heat for the process is supplied via the partial combustion of the feed; and PA1 c) combined cycle reforming consisting of both steam reforming and partial oxidation steps. PA1 1. U.S. Pat. Nos. 4,814,539 and 4,968,655 which disclose catalyst compositions containing a reducible compound on a silica-containing support; PA1 2. U.S. Pat. Nos. 4,992,409 and 5,053,578 which disclose catalytic compositions including a Group IA metal, a Group IIA metal and a third component, the precursor of which is a sol such as an aqueous suspension of a metal such as aluminum, silicon, titanium, zinc, zirconium, cadmium or tin with which the other components of the composition are thoroughly dispersed; and PA1 3. U.S. Pat. No. 5,028,577 which discloses a catalytic composition including a Group IA metal, a Group II metal, a third component the precursor of which comprises a sol (an aqueous suspension of aluminum, silicon, titanium, zinc, zirconium, cadmium or tin) and a fourth component including a Group VIII metal, silver or a combination thereof. PA1 a) a Group IIIB metal selected from the group consisting of yttrium, scandium and lanthanum; PA1 b) a Group IIA metal selected from the group consisting of barium, calcium and strontium; and PA1 c) a Group IVA metal selected from the group consisting of tin, lead and germanium, with the Group IIIB, Group IIA and Group IVA metals in an approximate mole ratio of 1:0.5-3:2-4, respectively, and a process employing same; and PA1 1. which is capable of promoting oxidative conversion reactions without requiring the addition of a catalyst layer onto the membrane and/or PA1 2. comprises a material with which the catalyst of such additional catalyst layer is compatible to facilitate manufacture,
Steam reforming involves the high temperature reaction of methane and steam over a catalyst to produce carbon monoxide and hydrogen. As the catalyst used in steam reforming can be very sensitive to the presence of impurities such as sulfur, the feed typically is appropriately pretreated to remove such impurities, e.g., such as by hydrodesulfurization.
While steam reforming produces a product containing lesser amounts of coke, the syngas produced typically has a high ratio of hydrogen to carbon monoxide (H.sub.2 /CO), usually in excess of 3:1. Also, steam reforming processing generally suffers due to the relatively large inputs of energy required to conduct the highly endothermic methane steam reforming reaction. In addition, the use and presence of steam in such reforming typically necessitates the use of costly special alloys for construction of the reaction vessel.
In partial oxidation processing, the large amounts of heat required in reforming processing of natural gas is advantageously supplied by the partial combustion of feed material. Such processing, however, generally suffers as, in addition to the desired products of carbon monoxide and molecular hydrogen, substantial undesired coproduction of carbon dioxide and/or carbon also occurs. In addition, such processing requires the use of pure oxygen typically obtained by the separation of air, such as through a cryogenic separation, a relatively expensive process.
In combined cycle reforming processing, an expensive air separation step is also typically required, although such processes can result in some capital savings since the size of the steam reforming reactor is reduced in comparison to a standard steam reforming process.
Thus, it is desirable to facilitate and lower the cost of syngas production as by, for example, reducing the cost of the oxygen plant, such as by reducing the cost of separating and obtaining oxygen from air. A process and a means whereby oxygen or another form of oxygen can be obtained from a relatively low cost source, such as air, and without incurring the relatively high costs associated with conventional oxygen separation techniques has been sought and is desired.
Another oxidative conversion process is commonly referred to as "oxidative coupling". Oxidative coupling has been recognized as a promising approach to the problem of effectively converting lower alkanes to higher molecular weight hydrocarbons. A mechanism of action of oxidative coupling processing, however, has not been clearly identified or defined and is not clearly understood. In oxidative coupling processing, a low molecular weight alkane, such as methane, or a mixture containing low molecular weight alkanes, such as natural gas, is contacted with a solid material referred to by various terms including contact material, catalyst, promoter, oxidative synthesizing agent or activator. In such processing, the methane is contacted with the solid material and, depending on the composition of the material, in the presence or absence of free oxygen gas, and is directly converted to ethane, ethylene, higher hydrocarbons and water. Carbon dioxide, the formation of which is highly favored thermodynamically, is an undesired product, however, as the formation of carbon dioxide results in both oxygen and carbon being consumed without production of the desired higher value C.sub.2+ hydrocarbons.
In such processing, catalytic mixtures containing reducible metal oxides are highly active but many such catalytic mixtures are undesirably very highly selective for producing CO.sub.2, that is, they are combustion catalysts. In order to obtain desired selectivity for hydrocarbon formation, Group IA metals, particularly lithium and sodium, have been used in the catalytic mixtures. Under the conditions used for oxidative coupling processing, however, migration and loss of the alkali metal typically occurs. Furthermore, in order to avoid complete combustion processing of the feed, many methods for such oxidative conversion have been carried out in the absence of oxygen-containing gas, relying on the oxygen theoretically being supplied by the catalyst (e.g., lattice oxygen). Nevertheless, in most cases involving oxidative coupling processing of methane, carbon monoxide and hydrogen are coproduced in addition to desired C.sub.2+ hydrocarbons.
Over the years, various additional oxidative coupling catalyst, contact agent, contact solid or the like compositions; additives, promoters, or the like for addition thereto and processes for oxidative coupling of hydrocarbons, particularly low molecular weight hydrocarbons such as methane, have been tested, reported or disclosed with varying degrees of success. Typifying these materials are those found in U.S. Pat. Nos. 4,444,984; 4,533,780; 4,547,607; 4,554,395; 4,567,307; and 4,568,785.
More particularly, U.S. Pat. Nos. 4,489,215; 4,495,374; 4,499,322; 4,499,323; 4,499,324; 4,450,310; 4,523,049; 4,656,155; 4,721,828; and 4,727,212 disclose processes utilizing compositions which, in addition to requiring, as a key component, a reducible metal oxide and/or depend on the utilization of lattice oxygen, include an alkali metal, an alkaline earth metal or combinations thereof.
For example, U.S. Pat. No. 4,495,374 discloses the use of a reducible metal oxide promoted by an alkaline earth metal in such a method of methane conversion. During such processing, the reducible metal oxide of the promoted oxidative synthesizing agent is reduced. The reduced synthesizing agent can then be removed to a separate zone wherein it is contacted with an oxygen-containing gas to regenerate the promoted oxidative synthesizing agent.
U.S. Pat. No. 4,523,049 shows a reducible metal oxide catalyst promoted by an alkali or alkaline earth metal, and requires the presence of oxygen during the oxidative coupling reaction. U.S. Pat. No. 4,656,155 specifies a reducible metal oxide in combination with an oxide of zirconium, an oxide of yttrium and, optionally, an alkali metal. U.S. Pat. No. 4,450,310 is directed to coupling promoted by alkaline earth metal oxides in the total absence of molecular oxygen. U.S. Pat. No. 4,482,644 teaches a barium-containing oxygen-deficient catalyst with a perovskite structure.
In addition, reference is made to the following commonly assigned U.S. patents concerned with the conversion of lower alkanes to higher molecular weight hydrocarbons via oxidative coupling/conversion:
In addition, commonly assigned U.S. Pat. Nos. 4,751,336; 4,754,336; 4,754,091 and 4,754,093 disclose oxidative coupling of lower molecular weight alkanes to higher molecular weight hydrocarbons utilizing a catalyst comprising silica free of a reducible metal oxide.
Several patents describe catalysts for higher hydrocarbon synthesis which can include a Group IIA metal; a metal of scandium, yttrium or lanthanum; and/or other metal oxides.
For example, U.S. Pat. No. 4,780,449 discloses a catalyst including metal oxides of a Group IIA metal, a Group IIIA metal, a lanthanide series metal excluding Ce, or mixtures thereof. The patent lists as optional promoter materials metal oxides of a metal of Groups IA, IIA, IIIA, IVB, VB, IB, the lanthanide series, or mixtures thereof.
Reference is also made to the following commonly assigned patents:
1. U.S. Pat. Nos. 4,939,311 and 5,024,984 which relate to a catalyst composition comprising a mixed oxide of:
2. U.S. Pat. Nos. 4,971,940 and 5,059,740 relating to a tin-containing composition and use, the composition comprising oxidized tin and having a specified tin Auger line transition.
Reference is also made to commonly assigned patent application U.S. Ser. No. 775,209 which discloses a contact material containing an intimately mixed, mixed oxide of at least one cationic species of a naturally occurring Group IIIB element, at least one cationic species of a Group IIA metal of magnesium, calcium, strontium, and barium and a cationic species of aluminum, as well as methods of hydrocarbon conversion using such contact material compositions.
The conversion of methane to higher molecular weight hydrocarbons in the presence of such solids, as described in the above patents, takes place at elevated temperatures in the range of about 500.degree. C. to 1,200.degree. C. The reaction is strongly exothermic in nature and in order to properly regulate the reaction and to prevent the occurrence of excessive undesirable side reactions, it is necessary to remove exothermic heat of reaction, avoiding an excessive temperature rise and lowering the temperature of the reaction product stream.
Fluidized bed reactor systems have been considered for the conversion of methane to higher molecular weight hydrocarbons. Fluidized bed reactor systems, however, generally suffer from problems of gas and solids back-mixing and gas bypassing with resulting losses in selectivity and yield. In addition, due to the high temperatures typically encountered in such processing and the large amounts of heat generated during such exothermic conversions, the physical structure of the reactor itself can be critical.
Problems particular to this conversion of methane include the fact that the reaction temperature is high enough to preclude or bring into serious question the use of many materials commonly used in reactor construction.
In recent years, workers in the field have proposed various reactor and processing configurations including some that make use of ceramic membranes.
For example, U.S. Pat. Nos. 4,791,079 and 4,827,071 disclose a multilayer membrane for use in hydrocarbon conversion, such as hydrocarbon oxidation and/or dehydrogenation processing. The multilayered membrane of these patents includes one layer which is an impervious mixed ion and electron conducting ceramic such as yttria stabilized zirconia which is doped with sufficient CeO.sub.2 or titanium dioxide to impart electron conducting characteristics to the ceramic. A second layer associated with the mixed ion conducting impervious ceramic is a porous ion conducting layer containing a selective hydrocarbon oxidation catalyst. These patents also disclose that in order to enhance oxygen dissociation, it may be desirable to further provide a thin layer of an oxide of lanthanum, chromium, tin, or the like on the surface of the membrane which contacts oxygen.
An oxygen-containing gas is contacted with the mixed ion and electron-conducting layer while a reactant such a hydrocarbon is contacted with the porous catalyst-containing layer with the system being maintained at reaction conditions. Oxygen ions pass through the mixed-conducting layer and catalytically react with the hydrocarbon in the porous catalyst containing layer. Product is separated from the porous layer while electrons pass through the mixed-conducting layer to balance the system.
U.S. Pat. No. 4,855,111 discloses a reactor with an annular reaction zone with ceramic baffles positioned therein perpendicular to the flow of a mixture of gas and fluidized solid catalyst. The baffles have openings adapted to permit passage of the gas solid mixture therethrough essentially only in the overall direction of flow from inlet to outlet. The patent states that known ceramic materials having sufficient strength at the elevated temperatures necessary for methane conversion can be employed. A ceramic material predominantly of alumina and which may contain minor amounts of oxides of silica, calcium and the like is disclosed as a preferred ceramic material.
U.S. Pat. No. 4,329,208 relates to the conversion of ethylene to ethylene oxide utilizing a solid electrolyte onto one surface of which is deposited an oxidation catalyst and onto a second surface of which is deposited a second catalyst capable of dissociating oxygen gas to oxygen ions. Oxygen ions are transported under a positive voltage applied through the solid electrolyte to react with ethylene to form ethylene oxide.
U.S. Pat. No. 4,599,157 relates to a selectively permeable composite membrane which permits oxygen gas to pass through while substantially blocking water vapor. In one embodiment, the selectively permeable composite membrane is a two-layer construction including a porous membrane layer having micropores and a thin layer containing, in a carbon matrix, a water-containable or wettable metallic oxide, a metal oxide having the capability of absorbing oxygen, or a metal oxide having a rutile-type crystal structure.
European Patent Application Publication No. 0 399 833 relates to a solid multicomponent membrane, electrochemical reactor and use of membranes and reactor for oxidation reactions. The membranes include substantially discrete phases of a) an electronically conductive material and b) an oxygen ion-conductive material and/or a mixed metal oxide having a perovskite structure. The electrochemical reactors are disclosed for use in a continuous process for transporting oxygen from an oxygen-containing gas to any reacting gas that consumes oxygen.
The electronically-conducting material or phase of the membrane is disclosed as being any material which exhibits sufficient electronic conductivity under the conditions of the reaction. Suitable metals and metal oxides for use in the electronically-conducting phase are disclosed as including silver, gold, platinum, rhodium, ruthenium, palladium, nickel, cobalt, copper, etc., among which palladium and platinum are preferred and bismuth oxides, tin-indium oxide mixtures, praeseodymium-indium oxide mixtures, cerium lanthanum oxide mixtures, niobium-titanium oxide mixtures, electron-conductive mixed metal oxides of a perovskite structure, etc., among which the metal-doped metal oxides are preferred.
The oxygen ion-conducting materials or phases of the dual-conductor are disclosed as typically being solid solutions formed between oxides containing divalent and trivalent cations such as calcium oxide, scandium oxide, yttrium oxide, lanthanum oxide, etc., with oxides containing tetravelent cations such as zirconia, thoria and ceria where the oxygen ion-conducting material or phases comprise an oxygen ion-conductive mixed metal oxide of a perovskite structure. Disclosed as preferred among the solid electrolites are the Y.sub.2 O.sub.3 -(yttria) and CaO-(calcia) stabilized ZrO.sub.2 -(zirconia) materials. Also, a wide variety of elements and oxides of elements are disclosed for use in forming perovskites useful in the invention thereof. Disclosed as examples of preferred elements are "La, Co, Sr, Ca, Fe, Cu, Ni, Mn, Or[sic], Y, Ba, Ti, Ce, Al, Sm, Pr, Nd, V, Gd, Ru, Pb, Na, W, Sc, Hf, Zr, oxides thereof, and mixtures thereof".
The document further discloses that the electrochemical cell may optionally contain a catalyst adjacent to or coated on the first conductive surface.
Also, Otsuka et al. in "Catalytic Activity and Selectivity Control for Oxidative Coupling of Methane by Oxygen Pumping Through Yttria Stabilized Zirconia," Chemistry Letters, pp. 319-322 (1985) describe the oxidative coupling of methane using electrochemically pumped oxygen through yttria stabilized zirconia having silver coated on one surface and silver-bismuth oxide coated on the other surface. In each case, the silver acted as an electrode which was necessary to complete the circuit external of the membrane and thus to permit the desired reaction to proceed. Otsuka et al. teach that the oxidative coupling of methane took place only when the circuit was closed by connection of lead wires from both electrodes.
Michaels et al. in "Kinetics of Vapor--Phase Electrochemical Oxidative Dehydrogenation of Ethylbenzene," Journal of Catalysis, 85, pp. 477-487 (1984) describe electrochemical oxidative dehydrogenation of ethylbenzene to styrene using an yttria stabilized zirconia ionic conductor in the form of a tube with porous platinum electrodes deposited in both inner and outer surfaces which are connected via an external circuit.
Stoukides et al. in, "The Effect of Electrochemical Oxygen Pumping on the Rate and Selectivity of Ethylene Oxidation on Polycrystalline Silver," Journal of Catalysis, 70, pp. 137-146 (1981) describe electrochemical oxidation of ethylene using an yttria stabilized zirconia ionic conductor having a porous silver catalyst film on both surfaces which films function as electrodes and are connected via an external circuit.
An important problem in many of the systems described above is the necessary provision of a catalyst layer deposited on the impervious ion conducting material which layer serves both as catalyst and as external electrode. It is difficult to secure such electrodes to surfaces of the ceramic membrane and to maintain the integrity of the electrode-membrane bond during sustained use at the severe conditions normally encountered. Without such electrodes connected externally, however, the desired electrochemical reaction did not proceed.
Thus, a membrane material which allows for the transport or conveyance of oxygen or a form of oxygen without requiring the attachment of an external electrode with the application of an electronic or electric potential to the membrane is desired. Further, in view of the problems associated with fabrication of membrane structures for the selective transport of oxygen, particularly those structures which contain a catalyst or the like to promote the desired oxidative conversion reaction, a membrane:
is desired.